Symptom: Runs out of fuel, Gauge inoperative or inaccurate.

Theory: These trucks have two fuel tanks on the driver’s side frame rail. The forward tank is the “main” and the rear tank is the “auxiliary”. There is also a fuel-pump-balance-module mounted on the inside of the driver side frame rail that monitors the fuel level in both tanks. When the fuel level in the front tank becomes 25% lower than the rear tank, the balance module transfers fuel from the rear tank to the front tank. It accomplishes this by running the “transfer pump”, also mounted inside the frame rail, ahead of the rear fuel tank. This pump may be referred to as a “secondary-pump” or “balance-pump” in some wiring diagrams.

The transfer pump only makes about 7psi and is only there to fill the front tank--it’s not meant to provide actual fuel pressure. The sender in the rear tank is for the balance module only, not for the gauge. The sender in the front tank provides information to both the balance module and the fuel gauge in the dash. Depending on vehicle and year, the fuel level output from the balance module will either go directly to the fuel gauge, or through the PCM/VCM and then on to the fuel gauge.

The Test: To properly diagnose this, you will need a Vantage/MODIS to check voltage and wave forms. You will also need a wiring diagram for your particular vehicle and will need to determine what the actual problem is with the vehicle. Is the gauge inoperative or inaccurate or does the vehicle run out of fuel even though there is still fuel in at least one tank?

If the gauge is wrong, you will need to take some measurements at the balance module. The signals from the sending units to the module and from the module to the fuel gauge are square waves on this system. Terminal-B (Light Blue) on the module comes from the rear tank sender, terminal-C (Blue/White) is the front tank sender and terminal-F (Purple) is the output to the fuel gauge or PCM/VCM. Set the Vantage/MODIS to Waveform Viewer and check the signals on these three terminals. The square-wave pattern will be taller, toward 12-volts, with more fuel in the tank and shorter, toward 0-volts, with less fuel. If you have an open circuit due to a bad sending unit or a bad ground, you will have a flat 12-volts but no wave pattern.

You can check the senders themselves at the balance-module connector. Unplug the module connector and, referring to your wiring diagram, check resistance between terminals E & B for the rear tank and terminals E & C for the front tank. In both cases, a full tank should result in about 90 ohms and an empty tank should read close to 0 ohms.

If the problem is that the vehicle won’t transfer fuel from the rear tank to the front, you still need to make sure the senders in both tanks are functioning properly. If they are, check to see if the balance module is triggering the balance pump relay. The relay should be mounted on or next to the balance module. Again, referring to the wiring diagram, monitor terminal-D (Light Green) at the balance module with the vehicle running. When the fuel level in the front tank is 25% less than the rear tank, the balance module should ground terminal-D.

You can manually run the transfer pump by temporarily jumping a ground to terminal-D of the balance module. This also needs to be done with the engine running, since power for the transfer pump relay comes from the same wire that powers the pressure pump. Be extremely careful when doing this, not only to make sure you ground the correct terminal, but if you leave the pump running too long, you can overfill the front tank causing a fuel spill.

Another way to check balance module operation would be to momentarily ground terminal-C, with the engine running, which will give a “low front tank level” signal to the module. If the balance module is functioning correctly, it should trigger the transfer pump relay, resulting in the transfer pump adding fuel to the front tank. Once again, caution is the rule--let it run too long and you’ll have gas all over the shop floor.

One last test would be to listen to the transfer pump. We have seen quite a few failures where it sounds like it’s grinding itself up inside and puts out little more than a trickle of fuel.

The Fix: Replace any parts found defective using the above tests.